AP Chemistry Unit 3 Spectroscopy: Light, Electrons, and Measuring Molecular Amounts

Spectroscopy

The study of how matter interacts with electromagnetic radiation (light), often through absorption or emission of specific energies.

Absorption (of light)

When a substance takes in light of certain energies because the photon energy matches an allowed energy change in the substance.

Emission (of light)

When a substance releases light of certain energies as it undergoes a decrease in energy (e.g., electrons relaxing to lower energy levels).

Electromagnetic radiation

Energy that travels as oscillating electric and magnetic fields; described as waves or as photons.

Quantized energy levels

Discrete (step-like) allowed energies in atoms/molecules; energy changes occur only in specific increments, not continuously.

Photon

A discrete packet of light energy; each photon carries an amount of energy determined by its frequency.

Wavelength (λ)

The distance between successive wave crests; typically measured in meters (or nm, then converted to meters for calculations).

Frequency (ν)

The number of wave cycles per second; measured in s⁻¹ (Hz).

Speed of light (c)

A constant in vacuum (~3.00 × 10^8 m/s) relating wavelength and frequency by c = λν.

Wave equation (c = λν)

Relationship showing that wavelength and frequency are inversely related because c is constant (longer λ means lower ν).

Planck’s constant (h)

Constant linking photon energy to frequency: h ≈ 6.626 × 10⁻³⁴ J·s.

Photon energy equation (E = hν)

Equation stating energy per photon is proportional to frequency.

Energy–wavelength relationship (E = hc/λ)

Equation showing photon energy increases as wavelength decreases.

Ultraviolet–Visible (UV–Vis) spectroscopy

Spectroscopy technique primarily involving electronic transitions; commonly used to measure concentration via absorbance.

Infrared (IR) region

Electromagnetic region that typically causes vibrational transitions; used for functional group identification via bond vibrations.

Nuclear Magnetic Resonance (NMR)

Technique using radio-wave-induced nuclear spin transitions to help determine molecular structure.

Photoelectric effect

Emission of electrons from a metal surface when light of sufficiently high frequency shines on it; supports the photon model of light.

Work function (Φ)

Minimum energy required to eject an electron from a given metal surface; a property of the metal.

Threshold frequency (ν₀)

Minimum light frequency needed for electron emission in the photoelectric effect; defined by Φ = hν₀.

Maximum kinetic energy (KEmax)

The greatest kinetic energy of emitted electrons in the photoelectric effect: KEmax = hν − Φ.

Light intensity (brightness)

Related to the number of photons hitting a surface per second; increases the number of emitted electrons (current) but not KEmax.

Beer–Lambert Law (Beer’s Law)

Linear relationship for many solutions: A = εlc, linking absorbance to molar absorptivity, path length, and concentration.

Molar absorptivity (ε)

Measure of how strongly a species absorbs light at a specific wavelength; depends on substance and wavelength.

Transmittance (T)

Fraction of light transmitted through a sample: T = I/I₀ (transmitted intensity over incident intensity).

Absorbance (A)

Logarithmic measure of light absorbed: A = −log(T); unitless and directly proportional to concentration via Beer–Lambert Law.